The present invention relates to means for converting electric power from either direct (D.C.) to alternating (A.C.) form or from alternating (A.C.) to direct (D.C.) form, and more particularly, it relates to an electrical power converting circuit that provides for control of the power flow in either direction between an A.C. system and a D.C. system.
A known inverting circuit is described in U.S. Pat. No. 3,406,327, issued Oct. 15, 1968 to N. W. Mapham et al, and entitled "Electric Power Inverter Having a Well Regulated, Nearly Sinusoidal Output Voltage". This patent describes a thyristor-controlled inverting circuit that operates to invert the voltage of a D.C. source to an A.C. output voltage. U.S. Pat. No. 3,406,327 further describes the A.C. output voltage as having a nearly sinusoidal waveform. Although the thyristor-controlled circuit of U.S. Pat. No. 3,406,327 serves well to accomplish its desired result, it is considered desirable in certain applications that a circuit be provided that performs both an inverting function, that is, it changes from D.C. to A.C., and a rectifying function, that is it changes from A.C. to D.C.
A circuit that performs both inverting and rectifying operations is commonly termed a converter, and one such converter is a "Graetz Converter". The Graetz converter is described in the text DIRECT CURRENT TRANSMISSION, Volume 1 of E. W. Kimbark published 1971 by John Wiley & Son. More particularly, the Graetz converter is generally described with regard to FIG. 1 of page 72 and FIG. 3 of page 132 of the Kimbark text.
FIG. 3 of page 132 of the Kimbark text shows the Graetz converter changing the D.C. voltage polarity but maintaining unchanged D.C. current I.sub.d polarity when it is controlled from the state of rectifying to the state of inverting. As stated by Kimbark on page 105 of his text with respect to the Graetz converter, "because the valves conduct in only one direction, the current in a converter cannot be reversed, and power reversal can be obtained only by the reversal of the average direct voltage V.sub.d ". This characteristic is disadvantageous if more than two converters are involved with a common D.C. system.
For example, with a plurality of more than two converters interconnected to a common D.C. system, each should be controllable to invert or to rectify with respect to that common D.C. system without interfering with the controlled state of inverting or rectifying of the other interconnected converters. However, if more than two Graetz converters are involved, appropriate switching of the converter busses to the D.C. system must be provided to allow the multiple converters to each perform the desired inverting or rectifying function. The switching operation implies either a partial or complete shutdown of the system and thus a disruption of the power flow. The switching is expensive; the switching operation is complicated; and the power flow disruption is highly undesirable. It is considered desirable that converters be provided for three or more terminal operations related to a common D.C. system having the proper flow of current so as to allow each of the multiple converters to perform its desired inverting or rectifying operation without causing a disruption of the power flow.
Accordingly, objects of the present invention are (1) to provide a converter circuit that performs an interchangeable inverting or rectifying operation without requiring or causing a disruption of the system power flow, even in a system including three or more converters, and (2) to provide a converter circuit that develops an A.C. current waveform that is nearly sinusoidal and that is restricted in its harmonic components.
Another object is to provide a converter that reverses the direction of D.C. system current flow through the converter when the operating mode of the converter is changed between inversion and rectification and which is capable of operating with a relatively high power factor when operating near full inversion or full rectification.
Still another object is to provide a converter capable of smoothly reversing the direction of the power flow between a D.C. system and an A.C. system and capable of smoothly adjusting the magnitude of such power flow between zero and a maximum value.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.